1. Field of the Invention
This invention relates to a process for upgrading a low value petroleum refinery stream, and more particularly to a process of converting petroleum residuum to distillate products and premium coke.
2. Description of the Prior Art
There are many processes available in the petroleum refining art for upgrading heavy, low value petroleum residual oils. Typical of such low value residual oils is the bottoms fraction from a vacuum distillation tower. Such vacuum distillation towers generally are used to further fractionate virgin atmospheric reduced crude oils. The bottoms fraction from such vacuum distillation columns generally includes all the material boiling above a selected temperature, usually at least 480.degree. C., and often as high as 565.degree. C. In the past, vacuum residuum streams have presented serious disposal problems, as it has been difficult to convert such streams to more valuable products in an economic manner. One method of disposing of vacuum residuum has been to use the stream as feedstock to a fluid bed or delayed coking unit. The resulting coke generally has value only as a cheap fuel. Fluid bed and delayed coking processes for converting vacuum residuum into coke are well known in the petroleum refining industry, and many commercial units utilizing these processes exist.
Another process which is available in the art for upgrading heavy, low value petroleum residual oils is hydrogen donor diluent cracking (HDDC). In this process a hydrogen deficient oil such as vacuum residuum is upgraded by admixing it with a relatively inexpensive hydrogen donor diluent material and thermally cracking the resulting mixture. The donor diluent is an aromatic-naphthenic material having the ability to take up hydrogen in a hydrogenation zone and readily release it to hydrogen deficient hydrocarbons in a thermal cracking zone. The selected donor material is partially hydrogenated by conventional methods using, preferably, a sulfur insensitive catalyst such as molybdenum sulfide, nickel-molybdenum or nickel-tungsten sulfide. Using this process, the heavy oil being upgraded is not directly contacted with a hydrogenation catalyst. Catalyst contamination by the heavy oil is thus avoided. Details of the HDDC process are described in U.S. Pat. Nos. 2,953,513 and 3,238,118.
Delayed coking of vacuum residuum generally produces a coke with a coefficient of thermal expansion (CTE) greater than 20.times.10.sup.-7 /.degree.C. The CTE of the coke is a measure of its suitability for use in the manufacture of electrodes for electric arc steel furnaces. The lower CTE cokes produce more thermally stable electrodes. Coke which is suitable for manufacture of electrodes for steel furnaces is generally designated as premium or needle coke. The CTE value required for a coke to be designated premium coke is not precisely defined, and there are many other specifications other than CTE which must be met in order for a coke to be designated premium coke. Nevertheless, the most important characteristic, and the one most difficult to obtain, is a suitably low CTE. For example, the manufacture of 61 centimeter diameter electrodes requires CTE values of less than 5.times.10.sup.-7 /.degree.C., and the manufacture of 41 centimeter diameter electrodes generally requires a coke having a CTE of less than 8.times.10.sup.-7 /.degree.C. Delayed coking of vacuum residuum from most crudes produces a coke with a CTE of greater than 20.times.10.sup.-7 /.degree.C., and such cokes, designated regular grade cokes, are not capable of producing a satisfactory large diameter electrode for use in electric arc steel furnaces.
As used herein, the term premium coke is used to define a coke produced by delayed coking which, when graphitized according to known procedures, has a linear coefficient of thermal expansion of less than 8.times.10.sup.-7 /.degree.C. Preferably, premium coke made according to this invention has a CTE of about 5.times.10.sup.-7 /.degree.C. or less.
Premium coke is produced commercially by delayed coking of certain refinery streams such as thermal tars, decant oil from a fluidized bed catalytic cracking operation for manufacture of gasoline, pyrolysis tar, blends of these materials, and these materials blended with minor amounts of vacuum residuum or other similar material.
Prior to this invention, there has been no process available which permitted the manufacture of premium coke from vacuum residuum, other than instances where a very small amount of vacuum residuum was blended with a conventional premium coker feedstock.
Premium coke is worth several times as much as regular coke. It is accordingly apparent that any process that can produce premium coke from a low value material such as vacuum residuum is much to be desired, and prior to this invention no such process was available to the industry.